1. A colorimetric method was developed for the direct chemical assay of human carboxypeptidase A (carboxypolypeptidase; EC with angiotensin converting enzyme-like activity in serum or plasma, with the substrate analogue glycyl-l-histidylglycine and the angiotensin converting enzyme substrate angiotensin I (A-I). This method was based on the spectrophototometric determination of histidylglycine and histidyl-leucine, products of the hydrolysis of glycyl-l-histidylglycine and A-I respectively. o-Phthalaldehyde reacted with the imidazole moiety of N-terminal histidyl peptides to produce a yellow chromophore.

2. A large number of inhibitors were tested for their effects on carboxypolypeptidase activity. The hydrolysis of Gly-His-Gly and A-I was inhibited by histidyl-leucine and angiotensin II, both products of the hydrolysis of A-I. Bothrops jararaca venom extract, EDTA, p-chloromercuribenzoate, 8-hydroxyquinoline and 2,3-dimercaptopropanol, previously reported as converting enzyme inhibitors, also inhibited carboxypolypeptidase activity.

3. Angiotensin converting enzyme activity in the serum of sixty-six adults ranged from 10 to 37 nmol of glycyl-l-histidylglycine hydrolysed in 10 min by 10 μl of serum at 37°C and pH 7·25.

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